The widths of electrical conductors, i.e., referred to as linewidths, on semiconductor substrates have been traditionally measured optically and/or with direct current (DC) probes. The optical measurement techniques have become increasingly difficult to use as linewidths have become smaller in integrated circuits. Expensive, complex optical apparatus is required for accurate measurement of micrometer and sub-micrometer linewidths.
Direct current techniques of linewidth measurement employ pointed electrical probes that make direct mechanical contact with probe pads electrically connected to interconnection lines having linewidths to be measured or to pads of a test structure having linewidths to be measured. Typically, a test structure is prepared at the same time interconnection lines are formed so that all lines of interest have the same width. Examples of techniques using patterns of conducting lines and electrical probes to determine the linewidths of conducting lines from measurements of current and voltage are described in U.S. Pat. Nos. 3,974,443 to Thomas, 4,516,071 to Buehler, and 4,956,611 to Maltiel. These electrical measurements of linewidths are subject to error, unless corrected, when linewidths become very narrow, i.e., in the micrometer to sub-micrometer range. Examples of test structures and appropriate correction techniques to account for very narrow linewidths measured electrically are described in Creswell et al , "Test Structure for the In-Plane Locations of Projected Features with Nanometer-Level Accuracy Traceable to a Coordinate Measurement System", 1993 IEEE International Conference on Microelectronic Test Structures, Barcelona.
An essential feature of conventional electrical measurement of linewidth is the direct mechanical contact between pointed electrical probes and pads extending from the lines having widths being measured. The probes are suspected of producing contamination when they contact and slide across probe pads in a linewidth test structure. Since cleanliness and the avoidance of contamination is a vital consideration in semiconductor device technology, it is desirable to avoid direct mechanical contact in measuring line widths.
Non-contact measurements of linewidths using capacitive coupling between a line or a line test structure and a capacitive probe have been suggested, but the capacitance required to achieve the coupling required for an accurate measurement is unreasonably large. Another non-contact method for measuring bulk resistivity of a conductive material that might be applied to measuring linewidths employs eddy currents. However, the amount of electrically conductive material present in the interconnection lines on a semiconductor substrate is too small for useful application of the eddy current method.
Although the known non-contact methods of measuring linewidths are not effective, it is still desirable to provide a method of measuring linewidths of interconnection lines on semiconductor substrates without direct mechanical contact with the electrical conductor of which the width is being measured.